This invention relates to a protected image and a process for the production of such an image.
International Patent Application No. PCT/US87/03249 (Publication No. WO 88/04237), the disclosure of which is incorporated herein by reference, describes a thermal imaging medium and a process for forming an image in which a layer of a porous or particulate image-forming substance (preferably, a layer of carbon black) is deposited on a heat-activatable image-forming surface of a first sheet-like or web material (hereinafter the "first sheet element"), the layer having a cohesive strength greater than its adhesive strength to the first sheet-like element. Portions of this thermal imaging medium are then exposed to brief and intense radiation (for example, by laser scanning), to firmly attach exposed portions of the image-forming substance to the first sheet element. Finally, those portions of the image-forming substance not exposed to the radiation (and thus not firmly attached to the first sheet element) are removed, thus forming a binary image comprising a plurality of first areas where the image-forming substance is adhered to the first sheet-like element and a plurality of second areas where the first sheet-like element is free from the image-forming substance. Hereinafter, this type of image will be called a "differential adhesion" image.
In a preferred embodiment of the imaging medium described in the aforementioned International Application, the image-forming substance is covered with a second laminated sheet-like element so that the image-forming substance is confined between the first element and this second element. After imaging and separation of the unexposed portions of the image-forming substance (with the second element) from the first element, a pair of images is obtained.
A first image comprises exposed portions of image-forming substance more firmly attached to the first element by heat activation of the heat-activatable image-forming surface. A second image comprises non-exposed portions of the image-forming substance carried or transferred to the second sheet element.
The respective images obtained by separating the sheets of an exposed thermal imaging medium having an image-forming substance confined therebetween may exhibit substantially different characteristics. Apart from the imagewise complementary nature of these images and the relation that each may bear as a "positive" or "negative" of an original, the respective images may differ in character. Differences may depend upon the properties of the image-forming substance, on the presence of additional layer(s) in the medium, and upon the manner in which such layers fail adhesively or cohesively upon separation of the sheets. Either of the pair of images may, for reasons of informational content, aesthetics or otherwise, be desirably considered the principal image, and all of the following discussion is applicable to both types of image.
The image-forming process described in the aforementioned International Application can produce high quality, high resolution images. However, the images produced by this process may suffer from low durability because, in the finished image, the porous or particulate image-forming substance, which is typically carbon black admixed with a binder, lies exposed on the surface of the image, and may be smeared, damaged or removed by, for example, fingers or other skin surfaces (especially if moist), solvents or friction during manual or other handling of the image.
It is known to protect various types of images by laminating transparent films over the image. For example, U.S. Pat. No. 4,921,776 describes a method of providing a lower gloss protective covering for a pre-press color proof. This method comprises laminating to the image surface a thin, substantially transparent integral polymeric film consisting essentially of a mixture of at least two slightly incompatible polymers, whereby the film exhibits a 20.degree. specular gloss that is at least 5% lower than the gloss of a film prepared from any one of the polymer constituents.
U.S. Pat. No. 4,902,594 describes a photoimaged article having a protected image composed of a colored image on a support, and a thin, transparent, flexible, non-self supporting, protective layer on the surface of the image. The layer is substantially non-tacky at room temperature, and has at least a major amount based on the weight of the layer of one or more thermoplastic resins of a vinyl acetal, vinyl chloride, or acrylic polymer or copolymer having a Tg of from about 35.degree. C. to about 110.degree. C. The layer is capable of being adhesively transferred directly to the image when the layer is first applied on the release surface of a temporary support, and the image and protective layer are laminated together under pressure at temperatures of between about 60.degree. C. to about 180.degree. C. with subsequent removal of the temporary support.
U.S. Pat. No. 4,719,169 issued Jan. 12, 1988 describes a method for protecting an image. This method comprises providing a colored image on a substrate and either:
a. applying an antiblocking layer to a release surface of a temporary support; bonding a thermoplastic adhesive layer to the antiblocking layer; laminating the applied support to the colored image via the adhesive; and peeling away the temporary support from the antiblocking layer; or
b. applying a thermoplastic adhesive layer to a release surface of a first temporary support; applying an antiblocking layer onto a release surface of a second temporary support, laminating the adhesive onto the colored image and peeling away the first temporary support; and laminating the antiblocking layer onto the adhesive layer and peeling away the second temporary support;
wherein the adhesive layer is substantially non-tacky at room temperature, is laminated at temperatures of about 60.degree. C. to 90.degree. C., and comprises one or more thermoplastic polymers or copolymers; and the antiblocking layer comprises one or more organic polymers or copolymers, which layer does not cohesively block at about 50.degree. C. or less. The intended use of this invention is to protect color proofs used in the graphic arts industry.
The protection of an image produced by the process described in the aforementioned International Application presents peculiar difficulties. A differential adhesion image has a microstructural or topographical character, with areas of the image-forming substance protruding above the sheet element to which it is attached (hereinafter called the "substrate"), and the surface characteristics of the image-forming substance are typically very different from those of the substrate. (If the imaging medium contains a release layer, as described in the aforementioned International Application, in some cases the areas of the image, which are not covered by image-forming substance, may have a surface formed of the release layer. Typically, the surface characteristics of this release layer are very different from those of a carbon black image-forming substance.) Furthermore, the porous or particulate image-forming substance used is typically more friable than, for example, printing ink, and thus more susceptible to abrasion, smearing and other deformation.
International Application No. PCT/US91/08345 (published as WO 92/09930 on Jun. 11, 1992) describes a process for protecting a binary image, such as that produced by the aforementioned International Application No. PCT/US87/03249, having a plurality of first areas, at which a porous or particulate image-forming substance is adhered to a substrate, and a plurality of second areas, at which the substrate is free from the image-forming substance. This protecting process is carried out by laminating to the image a laminating sheet comprising a durable layer and a support layer, with the durable layer facing the image, so that the durable layer adheres to both the first and second areas of the image. The support layer is then displaced away from the image such that the image remains covered with a durable layer which:
a) is substantially transparent;
b) has an abrasion resistance of at least 10 cycles of a 10 Newton force as measured by an Erichsen Scar Resistance Tester (referred to as an Erikson Abrasion Meter in the International Application No. PCT/US91/08345) and a critical load value of at least 100 grams as measured by ANSI PH1.37-1983; and
c) is not removed from the image by contact with adhesive tape having an adhesion to steel of 33 grams per millimeter as measured by ASTM D-3330.
The preferred durable layers for use in this process are acrylic polymers, and the process provides the binary images with protection adequate for many fields in which such images are used.
However, binary images having the specific durable layers mentioned in the International Application No. PCT/US91/08345 are not entirely satisfactory for use as copying media in the graphic arts industry. In this industry, it is common practice to position images securely in layouts with a strong adhesive tape (hereinafter called "graphic arts tape", and also referred to in the industry as "ruby tape"; one major brand is sold commercially as "Red Lithographers tape #616" by Minnesota Mining and Manufacturing Corporation, St. Paul, Minn., 55144-1000), and it is frequently necessary to secure an image with such tape and later to peel the tape from the image, and then to repeat this process several times. Also, in this industry images are subject to multiple washings with isopropanol and other solvents to ensure the high degree of cleanliness needed in images used for further copying. It has been found that under the extreme stresses caused by such repeated applications of graphic arts tape and repeated washings, the durable layers mentioned in the International Application No. PCT/US91/08345 may not adhere adequately to the underlying image. Accordingly, there is a need for protection of such binary images in a manner which renders the protected image durable, transparent and abrasion-resistant, and permits repeated applications of graphic arts tape, and repeated solvent washings of the protected image, without risk of separation of the durable layer from the binary image. This invention provides a process for such protection of binary images.